Maternal changes in pregnancy

- Pregnancy is the development of offspring inside a woman's uterus. It's also called gestation, and during gestation a lot of changes occur in mom's body. So the average pregnancy lasts about 38-1/2 weeks, and for ease of discussion when we're talking about pregnancy, we sort of talk about it as having three phases, called trimesters, and each trimester lasts about 13 weeks. So if I said that event X happens in the first trimester, you'd know I meant some time between one and 13 weeks of pregnancy. And if event Y happens in the second trimester, that would be between weeks 14 and 26 and so on. So, three trimesters. So during the second and third trimesters the fetus is sort of getting a lot bigger and more developed, and so the mother's uterus has to expand to accommodate the growing fetus, and actually both the fetal growth and the uterine growth cause a lot of physiological change in the mom. So before we go on to talk about some of the non-reproductive system physiological changes in the mother, you must be wondering well, how exactly does the uterus grow so much? So let's take a look at that first. So it's really important to note that basically all of the maternal changes that happen during pregnancy, including this uterine growth, can be attributed in some way to hormones, especially to the ones we classically think of as those female sex hormones, the estrogen and the progesterone. The hCG or human chorionic gonadotropin from the embryo, that plays a pretty big role as well. So at the start of pregnancy, until about week 12, so almost until the end of the first trimester, the corpus luteum and the ovary produces most of the estrogen and progesterone that initially support the embryo and cause the development of the placenta. But during the first part of the second trimester the corpus luteum degenerates, and the placenta takes over as the primary hormone producer, as the new major endocrine organ of the pregnancy. And remember, endocrine organs secrete hormones to regulate body function. So the placenta takes over as the major endocrine organ, and it starts to create estrogen that'll actually have some major effects in both the mother and the fetus. And actually, the placenta just kind of keeps pumping out estrogen throughout the pregnancy, so the estrogen levels just continually rise until the end of the pregnancy at around the 38-1/2 week point. So all this estrogen around, what exactly is it doing? Why is it necessary? Well, I'll just mention a few reasons. First, it's suppressing FSH and LH release from the anterior pituitary by negative feedback. This is really important because what happens when FSH and LH are floating around in the blood? Well, we start getting development of follicles and eggs in the ovaries, right? And we can't really afford to have ovulation happening during the pregnancy. The mother already sort of has enough on her to do list with supporting an existing pregnancy. Second, the high levels of estrogen are really important in inducing growth of the fetus' organ systems, like the fetal lungs and liver. Third, the estrogen sort of stimulates maternal tissue growth. So the uterus enlarges to accommodate the growing fetus, and the ducts and the mammary glands and the breasts expand and branch in preparation for lactation and breastfeeding. So that's estrogen during pregnancy. The anterior pituitary gland gets a little bit bigger during pregnancy as well, and it starts to make more thyrotropin, among other things. Thyrotropin affects the thyroid gland and increases mom's metabolic rate, and that in turn increases her appetite and can cause the hot flashes associated with pregnancy. And the reason mom's metabolic rate and her appetite increase is because it takes a lot of energy to support gestation, and so it's important that mom gets enough food to meet those increased energy needs. There are some other major physiologic changes that happen too, though. For example, in the circulatory system blood volume has to increase by an extra one to two liters of blood to make sure the fetus is nourished properly. So because of this extra blood volume, the mother's blood pressure and pulse rise during pregnancy, and also as the uterus grows, it eventually will compress the pelvic blood vessels underneath it, and that'll obstruct venous return to the heart from the pelvis and the legs, and so among other conditions, this can cause varicose veins to develop. There are some skin changes as well. The expanding uterus also stretches the overlying dermis on the abdomen, dermis meaning skin, and when the dermis gets stretched too far beyond what it can handle, connective tissue within and underneath the dermis can tear and cause striae or stretch marks. So typically you'll see these reddy-purply striae in places that get stretched during pregnancy. So besides the abdomen, you might see these striae on the breasts, on the thighs, and on the hips. They usually fade to a pretty faint color a few months after childbirth, though. The lungs have a bit more work to do during pregnancy as well. In fact, they've gotta work about 50% harder, and this is to provide oxygen to all that extra blood that mom's got on board for the fetus. But to sort of complicate things a bit, the growing uterus pushes up on mom's diaphragm, preventing her from taking in full breaths. So she might develop a bit of shortness of breath during the later trimesters. The uterus puts a bit of pressure on the stomach as well, and that can cause some heartburn or gastric reflux in mom, but luckily in the last few weeks of pregnancy the fetus sort of descends a bit in the pelvis, and that reduces the upward pressure on the diaphragm, so mom can breathe a bit better, and the heartburn typically goes away as well. The flip side of this, though, is that the uterus, because it's dropped a bit, it then puts even more pressure on the urinary bladder, and that leads to needing to urinate a lot more frequently. We haven't really talked about the weight gain in pregnancy, and there's a fair amount of extra weight that the mother has to sort of lug around with her, and the weight gain during pregnancy comes from a few different things. The growing fetus is generally the biggest portion of it, about seven to eight pounds. The placenta and the amniotic fluid add about six pounds, and the extra breast tissue adds about three to four pounds. The growth of the uterus itself adds about four pounds, and the extra blood volume adds about four pounds as well. Actual fat or adipose tissue gain varies the most, but only really needs to be between three and 10 pounds. So the total typical weight gain during pregnancy is somewhere around 25 to 35 pounds. So those are some of the major body system changes that happen in mom's body during pregnancy.